corridor-a route comprising the same habitats as are present at either end of the route, thereby giving all species a chance to move through it;
• filter-a route comprising only some of the habitats available at either end of the route, thereby preventing certain species that depend on the absent habitats from moving;
• sweepstakes-a route comprising habitats which are absent from the source of species and may represent, in analogy or actuality, a sea separating two areas of land. The chances of a species crossing this 'sea' are very small but, under certain conditions of wind or the stranding of individuals on a raft of floating vegetation drifting down a river and out to sea, a terrestrial species might successfully cross over to another area of suitable habitat. For example, after heavy rains around Manado in 1882, large forest trees were seen floating out to sea, each sufficiently large to provide a temporary home for a few small mammals such as squirrels and invertebrates. Lava can also push trees into the sea. In these two cases one might expect agile animals to jump off the trees, but a few days after the Krakatoa eruption in 1883, a monkey was found, scorched and tired, clinging to a partly-burned tree in the Sunda Straits. The monkey was kept alive for at least a year (Hickson 1889). Even if an animal is washed alive on to a new coast, if there is no member of the opposite sex available then the sweepstakes will have been lost.
So there are three ways in which an organism can disperse:
• by 'jumping' quickly over relatively large expanses of unsuitable habitat;
• by dispersing slowly across habitat which is more or less suitable; and
• by dispersing very slowly and making adaptations on the way allowing the colonizing of areas the environments of which would have been unsuitable for the original stock.
It is well understood by laymen and scientists alike that small islands support fewer species than large islands. After a certain length of time the total number of species on an island will remain more or less constant, and this represents an equilibrium between the colonizing of the island by immigrant species and the extinction of existing species. The rate of colonization is clearly higher when an island is near the mainland because more species are likely to cross the relatively narrow sea gap. Also, the rate of extinction is clearly greater when an island is smaller because the population of any species will be smaller and the chance will be greater of disease and other detrimental events reducing the population to zero or an unviable number. These relationships can be drawn graphically and represent the foundation of the Theory of Island Biogeography (fig. 1.25).
The relationship between island size and number of species is relatively constant for a given group of animals or plants, and in general reducing island area by a factor of ten, halves the number of species (figs. 1.26 and 1.27). Where an island supports fewer species than expected and so falls below the line, the reason may be that:
• the group is not sufficiently well known;
• equilibrium in species number has not yet been reached (where a volcanic island such as Una-una has been destroyed and is being recolonized);
• the island comprises a relatively restricted number of habitats, or habitats which do not support large numbers of species;
• the island is extremely remote and difficult to colonize.
Figure 1.25. The relative number of species on (a) small, distant islands (b) large, distant or small, close islands, and (c) large, close islands.
Where an island supports more species than expected and so falls above the line, the reason may be that:
• more than the equilibrium number are present and some species will in due course be lost;
• the island is peculiarly rich in habitat types; or
• the island is a centre of species radiation in a certain group.
For total species, Sulawesi is below the line of best fit for both plants and birds. For plants this is probably because there has been insufficient collecting (p. 29), but this is unlikely to be the reason for the birds. Sulawesi falls above the line for mammals due, perhaps, to the extraordinary radiation of rat species, and is more or less on the line for snakes. Sulawesi is consistently above the line for the number of endemic species and this reflects its geological history.
These principles are useful in deciding whether an area is well-known biologically. For example, plotting known number of species of milkweed butterflies (Danaidae)23 against island size (by rank) reveals that islands around Southeast Sulawesi and the Togian Islands have probably not been surveyed sufficiently for this group, whereas for the Banggai Islands and islands in South Sulawesi, nearly all the species of this family are probably known (fig. 1.28).
Figure 1.26. Relationship between number of species and island size for revised plants (those with a recent taxonomic review) and three groups of animals.
From Anon. 1982b
When lists of species found on large and small islands are compared, it is generally found that the species absent from the smaller islands are larger than average since these animals generally have large range requirements and low densities. A few species are more abundant and fill a wider niche24 on islands than they do on the neighbouring mainland or larger islands where they have more competitors although this relationship does not always hold (MacArthur et al. 1972).
Figure 1.27. Relationship between number of endemic species and island area for revised plants and mammals, birds and reptiles.
From Anon. 1982b
Figure 1.28. Number of species of milkweed butterflies found on Sulawesi islands.
After Ft. Vane-Wright (pers. comm.)
Wallace's Line
It is an enigma that educated people in Indonesia know about Charles Darwin, even adopting his surname, and yet know nothing about Alfred Wallace, his contemporary and and fellow Englishman. Darwin is usually given credit for formulating the theory of evolution by natural selection, but the primary reason he was encouraged to write down his thoughts was because Wallace had written to him from Indonesia enclosing a manuscript showing that he had reached more or less the same conclusions as Darwin. Both men had been stimulated by what they had seen while travelling, and the interpretation of their observations did not match with the contemporary wisdom concerning the creation of the world. Wallace had been particularly fascinated by a visit to Sulawesi and the first manuscript he sent to Darwin was written in Ternate after leaving Manado. The thoughts of these two men on the distribution and evolution of species turned modern thinking upside-down and Wallace's contribution should not be underestimated.
Whereas Darwin was well-educated and came from a rich family, Wallace left school at 14, in 1837, and eventually earned a living by collecting animals in remote areas of the world to sell to museums. He spent eight years in Sarawak and Indonesia (from Sumatra to Irian Jaya) and the account of his travels totalling about 22,000 km makes splendid reading (Wallace 1869).
In a letter written in 1858, Wallace
